Effects of changes in arterial carbon dioxide tension on oxygen consumption during cardiopulmonary bypass

The effects of changes in arterial carbon dioxide tension (PaCO2) on the oxygenation of tissues in 34 patients undergoing surgery for aortocoronary bypass were studied while temperature, systemic blood flow, and the delivery of oxygen to the peripheral tissues remained constant. Mixed venous and superior vena caval oxygen tensions (PvO2 and PsvcO2, respectively) and oxyhemoglobin saturations and the in vivo partial pressure of oxygen at which 50 percent of the hemoglobin is saturated (P50) increased with PaCO2, while peripheral vascular resistance, in vitro P50, the level of 2,3-diphosphoglyceric acid in the red blood cells, and the level of lactate in the blood remained constant. There was a close correlation between increases in PaCO2 and increases in PvO2 (r = 0.912; P less than 0.001) but not increases in PsvcO2 (r = 0.364; not significant). This indicated that the total-body consumption of oxygen diminished with increases in PaCO2 but that some regional redistribution of oxygen consumption occurred between the superior and inferior vena caval vascular beds. Since the level of lactate in the blood remained constant and since signs of metabolism acidosis did not develop, the reduced oxygen consumption due to increases in PaCO2 did not result in detectable increases in anaerobic metabolism.